1. Field of the Invention
The present invention relates to a thin film transistor including titanium oxides as an active layer, and especially to its structure and a method of manufacturing the same, which can be used for low-cost production of a thin film transistor, have a potential to solve harmful environmental problems, improve the performance of a thin film transistor, and can be widely applied to specific electronic apparatuses.
2. Description of the Related Art
Recently, multipurpose displays, which are lightweight, can be increased in size on demand, and can be reduced for easier storage when they are not in use, have attracted a considerable attention in consideration of the benefits such as portability and bidirectional communication. In particular, flexible displays based on organic electroluminescence displays (OLEDs), liquid crystal displays (LCDs), electrophoretic displays, and the like are being developed.
Organic thin film transistors are particularly attracting attention as driving circuitries in such flexible displays. Organic semiconductors used for organic thin film transistors are mechanically flexible compared to silicon semiconductors. Organic semiconductors have a charge mobility of about 0.001˜several ones cm2/Vsec, and thus are limitedly used only in displays that do not require much current.
Organic thin film transistors have limited current driving capability due to their limited charge mobility. The current driving capability of organic thin film transistors can be increased to some degree by increasing the ratio of width to length in a channel, but, in this case, there is a problem in that the aperture ratio, which is the ratio of light-emitting area to the total area of a pixel, decreases.
Furthermore, since organic semiconductors are weakly resistant to oxygen or moisture in the air, it is difficult to realize perfect packaging. Since a flexible substrate has much higher transmissivity of oxygen and moisture than a glass substrate, etc., it is even more challenging to realize a perfect packaging for flexible displays using organic thin film transistors.
Meanwhile, important transistor technologies include amorphous silicon transistors and low-temperature polysilicon transistors. The amorphous silicon transistors are problematic in that they have low charge mobility and their threshold voltages drift over time. Low-temperature polysilicon transistors are also problematic in that they cannot be used together with plastic substrates that must be handled at temperatures much lower than the processing temperature of low-temperature polysilicon.
In order to overcome the above problems, transistors based on metal-oxide semiconductors, particularly those based on zinc oxide (ZnO) or its compounds, have been proposed. However, in order to realize a high charge mobility in such metal-oxide transistors, expensive metals, such as indium (In), tin (Sn), gallium (Ga), and the like, are added thereto.
Such metal-oxide thin film transistors (MOxTFT) have lately attracted considerable attention as alternatives to silicon semiconductor-based TFTs and organic semiconductor-based transistors because they can realize an ideal balance between cost, performance and processability in the application fields like active matrix displays, radio frequency identification (RFID) tags, and the like. Further, these MOxTFTs are expected to be applied to new applications in which transparency is required or desired.
However, as described above, almost all MOxTFTs have been fabricated based on ZnO or mixtures thereof to date. Moreover, since most successful MOxTFTs are mixed with indium, there is a problem in that the material costs and processing costs are high. This presents a great impediment to use in the fields of extremely low-priced goods, such as RFID tags and the like.
Therefore, efforts have to be made to find novel base materials for realizing thin film transistors with excellent performance and suitable prices and to develop the structures of thin film transistors using the found base materials and methods of manufacturing the same.